Vehicle headlamps commonly include a lamp capsule mounted in a reflector so that the light source is located at or near the focal point of the reflector. Light emitted by the lamp capsule is directed in a forward direction by the reflector. The lamp capsule typically includes a high beam filament from which light is directed horizontally in a high beam pattern and a low beam filament from which light is directed below horizontal in a low beam pattern. One of the problems involved in the design and construction of vehicle headlamps is to minimize uncontrolled light emission outside the desired beam patterns, particularly the low beam pattern, that may impair the ability of oncoming drivers to see the road and other vehicles. This uncontrolled light is known as glare.
The typical low beam pattern requires that little or no light be projected above the horizontal plane and requires the maximum hot spot to be projected just below the horizontal plane. Factors such as filament size and internal and external uncontrolled reflections cause the source pattern to be spread, making a sharp transition line at the horizontal plane difficult to achieve. One approach is to aim the low beam pattern slightly lower than horizontal. This reduces glare for oncoming drivers but at the expense of reducing desired illumination far down the road. Another approach is to block undesired light. However, any blockage reduces the total illumination produced by the lamp and thereby reduces the effectiveness of the lamp. There is therefore a need to improve the low beam pattern and in particular to sharpen the transition line at the horizontal plane, while minimizing the adverse impact on the total illumination.
In a two-filament lamp capsule, light from the low beam filament falls on the high beam filament at close range, causing it to appear as if the high beam filament were illuminated at low level in the direction of the low beam filament. Regions not facing the low beam filament remain dark. Light reflected from the high beam filament is then projected onto the reflector and into the field of view. The light reflected from the high beam filament is projected by the reflector as if the high beam filament were partially illuminated and produces a ghost image of the high beam filament. Thus, light is projected above the horizontal plane into the region that should not receive light when the low beam filament is illuminated. It is desirable to reduce or eliminate this ghost image of the high beam filament during low beam operation without substantially affecting lamp intensity during high beam operation.
Vehicle headlamps include a filament support structure which supports the high beam and low beam filaments in desired positions in the lamp capsule and which conducts electrical energy to the filaments. The filament support structure typically includes conductive support leads having sufficient rigidity to support the filaments under all expected environmental conditions. The filament support structure should be configured to limit blockage of light emitted by the filaments and to limit stray reflections that would adversely affect the beam pattern. Furthermore, the positions of the filaments in the lamp envelope and relative to each other have a significant impact on the beam pattern and on the overall performance and flexibility of the lamp capsule.